1. Field of the Invention
The present invention is related generally to a circuit for image displacement in a particle beam apparatus independently of magnification and more particularly to such a circuit including a negative feedback current amplifier at its input side which is charged with a time dependent signal for deflecting a particle beam and whose output signal is applied to an input of a deflection unit of the particle beam apparatus.
2. Description of the Related Art
For magnified imaging of a specimen in a scanning electron microscope, the surface is scanned line-by-line with an electron beam. This triggers secondary electrons on the specimen, the number of secondary electrons being dependent upon, among other things, the material composition and the surface structure at the point of incidence of the specimen. The strength of the secondary electron current emanating from the specimen and modulated by the material contrast and topography contrast is measured in a detector whose output signal controls the intensity of a sweep beam of a monitor which is deflected synchronously with the electron beam.
In a scanning electron microscope, the magnification factor V is defined by the relationship ##EQU1## The magnification is adjustable by the current flowing in the deflection coils and covers several orders of magnitude. To select the deflection current and, thus, the magnification in the desired range, negative feedback current amplifiers are usually used. Such negative feedback amplifiers generally derive their signals from precise resistors which are switchable in a plurality of steps. For electrical image displacement, in other words to displace the scan field on the specimen surface, a variable constant field is superimposed on the field of the deflection coils. The simplest solution generates the desired constant or continuous field by adding a dc voltage to the input of the current amplifier. This has the disadvantage that the image displacement is dependent upon the magnification V which is set.
In known arrangements, one is therefore forced to modify the dc voltage or its feed or supply every time there is a switch of the precision resistors. This becomes technologically involved and cannot always be exactly implemented, resulting in discontinuous image displacements when switching magnification.
Generally, the circuit shown schematically in FIG. 1 for image displacement in a scanning electron microscope independently of magnification includes a negative feedback current amplifier OV, a coil pair SC for deflection of an electron beam PE on a specimen PR, a plurality of precision resistors R.sub.M selectable by a switch S.sub.l for setting the size of a scan field RF on the specimen surface, and a voltage source U.sub.V whose dc voltage is superimposed by one of a plurality of resistors R.sub.V on a saw-tooth deflection signal U.sub.S at an inverting input of the operational amplifier OV. Since the magnetic field established in the beam path of the primary electrons PE with the assistance of the deflection unit determines the deflection of the electron PE on the specimen surface, the size of the scan field RF in the direction of the arrow is determined by the current flow in the coils SC. The relationship EQU i={R.sub.r /(R.sub.e.R.sub.M)}U.sub.S +{R.sub.r /(R.sub.v.R.sub.M)}U.sub.v :=i.sub.S +i.sub.B, (2)
is valid for the intensity of the current i, where i.sub.S defines the size of the scan field RF in the direction established by the coil field and i.sub.B defines the image displacement in a deflection direction which is adjustable by the voltage source U.sub.V. In otherwords, the image displacement refers to the position of the scan field R.sub.F on the specimen PR.
As equation (2) shows, the direct current i.sub.B which effects the image displacement is also dependent on which one of the precision resistors R.sub.M is selected by the switch S.sub.1. To guarantee an image displacement independently of the magnification V, and therefore independently of the respective precision resistor R.sub.M, one is forced to switch the resistor R.sub.V using a switch S2 which is coupled to the switch S1 since a change in magnification is provided by the product (R.sub.V.R.sub.M) and, thus, the direct current i.sub.B retains the established value.